This invention relates generally to the field of non-destructive examination, and more specifically to the non-destructive examination of portions of a steam turbine apparatus, and particularly to an apparatus and method for the remote inspection of the bell seal area of a high pressure steam turbine.
Steam turbines are well known in the power generation industry. A steam turbine is a device operable to extract energy from a flow of high pressure, high temperature steam and to convert that energy into mechanical energy in the form of the rotation of a shaft. The steam flow may be generated by any known type of steam generator, such as for example, a fossil fueled boiler or a nuclear steam supply system. The rotating shaft of the turbine is commonly connected to a rotor shaft of an electrical generator to convert the mechanical energy of the rotating shaft into electrical energy for distribution via the power grid.
A typical steam turbine is illustrated in FIG. 1. The steam turbine 10 includes a rotor shaft 12 journaled for rotation within an inner cylinder 14 and an outer cylinder 16. The inner cylinder 14 includes, among other parts, a blade carrier ring 18 and several nozzle chamber units 20 each welded to the inner cylinder so as to become an integral part thereof. The outer cylinder 16 includes one or more high pressure steam inlets 22 and a number of inlet sleeve units 24, each of which extends inwardly in telescoping relation to its associated nozzle chamber 20 in the inner cylinder 14. Steam enters the turbine inlet 22 from a high pressure steam line (not shown) downstream from one or more control valves (not shown) into a nozzle chamber 20 integrally attached to the inner cylinder 14. The steam then passes through the nozzles and rotating blades 26 of the control stage, which are attached to the rotor shaft 12. Steam flows from several parallel inlet paths flows into a control stage chamber 27 and around the nozzle units 20 to merge together to flow through the remainder of the turbine array of stationary 28 and rotating 29 blade rows. The expanded steam exiting the blade rows enters a steam outlet annulus 36 formed between the inner and outer cylinders 14, 16 and is directed to an outlet 38.
The inlet steam flow must pass between the inner and outer cylinders 14, 16 without leakage there between. This requires a static seal that will withstand extremely high pressures, high temperatures, and differential thermal expansion. The seal must be substantially fluid tight and stable under conditions of extremely high velocity and sometimes pulsating steam flow. The seal assembly repeatedly encounters dynamic instability, vibration, and thermal shock during use. It is know to use a bell seal 30 for this application. Several known designs of such bell seals are described in U.S. Pat. No. 3,907,308 dated Sep. 23, 1975; U.S. Pat. No. 4,802,679 dated Feb. 7, 1989; and U.S. Pat. No. 4,812,105 dated Mar. 14, 1989.
Reliable operation of a steam turbine is desired in order to ensure the integrity of the electrical power supply and to avoid unplanned and therefore more costly repairs resulting from failures during the operation of the turbine. A variety of routine inspections are performed on a steam turbine to assess the condition of the machine during its useful operating life, and to detect degraded conditions before they mature into a component failure. The inlet sleeve area of a turbine is subject to extremes of temperature, thermal shock, vibration, and differential expansion, and as such, is an area vulnerable to mechanical wear and cracking. In particular, it is known that the surface 32 of the inner cylinder 14 in contact with the bell seal is subject to wear. Such wear can result in a decrease in the effectiveness of the bell seal and a greater leakage between the inner cylinder 14 and the outer cylinder 16 than is desired. Furthermore, the trepan radius area 34 of the outer cylinder inlet sleeve 24 has been known to develop high cycle fatigue cracks in some turbines.
It is known to inspect portions of a steam turbine by inserting a miniature camera into the turbine through the main steam inlet nozzle 22, such as is taught by U.S. Pat. No. 5,164,826 dated Nov. 17, 1992. However, inspections of the bell seal and trepan radius areas 30, 34 have previously been performed with the turbine out of service and with the turbine casing disassembled to provide access to these parts. Once the turbine is disassembled, the bell seal 30 may be visually inspected and measured for wear. The known techniques for the inspection of the bell seal of a steam turbine are time consuming and expensive because they involve the disassembly of the turbine. Consequently, these inspections are generally performed only during scheduled turbine maintenance outages when the turbine is being disassembled for other purposes
Thus there is a particular need for an inspection technique that provides a non-destructive examination of the inlet sleeve bell seal without the need for the disassembly of the turbine. Accordingly, a method for inspecting the inlet sleeve bell seal of a steam turbine is described herein, the method including the steps of providing a tool having a pair of bladders spaced along a guide tube; inserting the tool into a turbine inlet to an inspection position wherein a turbine bell seal is disposed between the pair of bladders; inflating the pair of bladders to form a sealed volume having a leakage path through the bell seal; establishing and varying a flow of pressurized air into the sealed volume; and recording data representing the pressure in the sealed volume as a function of the rate of flow of pressurized air; comparing the recorded data to predetermined standard data to determine a condition of the bell seal. The method may also include the steps of providing a camera on the tool; monitoring the output of the camera during the step of inserting the tool into a turbine inlet to determine when the tool is in the inspection position; and monitoring the output of the camera to perform a visual inspection of the bell seal.
A method of testing the inlet sleeve bell seal of a steam turbine as described herein may alternatively include the steps of generating an algorithm for predicting the degree of degradation of a bell seal based upon the mass flow rate of air flowing through the bell seal under various pressure conditions; providing an inspection tool adapted for measuring the mass flow rate of air flowing through a bell seal installed in a turbine; inserting the inspection tool into an assembled steam turbine through an opening; obtaining measurements of the mass flow rate of air flowing through the bell seal of the turbine under various pressure conditions by operating the inspection tool; and predicting the degree of degradation of the bell seal of the turbine by apply the measurements to the algorithm.
An apparatus for in-situ inspection of a bell seal of a turbine is described herein as including a guide tube adapted for insertion into an inlet of a turbine; a pair of inflatable bladders attached to the guide tube and separated by a space sufficient to span a bell seal of the turbine; an inspection air flow path having an outlet disposed between the pair of bladders; a flow sensor for indicating the rate of flow of air through the inspection air flow path; and a pressure sensor for indicating the pressure in a space between the bladders. The apparatus may further include a camera connected to the guide tube and disposed between the pair of inflatable bladders, and a motor connected between the camera and the guide tube for rotating the camera about a longitudinal axis of the guide tube.
An inspection apparatus that may be used for performing the disclosed method may include a guide tube having an insertion end; a trailing bladder connected to the insertion end of the guide tube; a leading bladder spaced apart from and connected to the trailing bladder; a means for selectively inflating each of the trailing bladder and the leading bladder; a means for providing a variable flow of air to the space between the leading bladder and the trailing bladder; a means for measuring the rate of flow provided by the means for providing a variable flow of air; and a means for measuring the static air pressure in the space between the leading bladder and the trailing bladder.